Refrigeration systems are employed to produce cooling of a product or an environment. To do this cooling they abstract heat from that product or environment at a lower temperature level and reject the abstracted heat at a higher temperature level along with the heat equivalent of the thermal or mechanical energy utilized to move the abstracted heat from the lower to the higher temperature level. Though there are many types of refrigeration systems, the most common type uses a volatile refrigerating fluid, or refrigerant, circulated in a sealed system. The sealed system includes the following components:
A first heat transfer element or evaporator for boiling liquid refrigerant to a vapor thereby cooling the product or environment adjacent the evaporator, A compressor, conduit connected to the evaporator to draw vapor from it, thereby lowering the pressure in the evaporator. The compressor discharges the compressed vapor at an elevated pressure to a PA1 second heat transfer element or condenser which rejects heat from the compressed vapor to an air or water ambient, thereby condensing the compressed vapor to a liquid. The liquid refrigerant is returned to the evaporator by a conduit known as PA1 a liquid line to repeat the cooling effect. Before the liquid enters the evaporator it passes through a restrictor which may be in the form of an automatic control valve usually known as an expansion valve. When the expansion valve is designed to respond to temperatures at the vapor outlet, also called suction outlet, of the evaporator, it is called a thermal expansion valve or TXV; In small systems the restrictor is not adjustable and is in the form of a tube with a tiny bore known also known as a capillary tube or cap tube. The restrictor, whatever its type, lowers the pressure of the liquid refrigerant from the higher pressure found in the condenser to the lower pressure found in the evaporator and also regulates the flow of the liquid refrigerant into the evaporator to exactly that amount which the evaporator can evaporate. PA1 In larger systems a holding tank called a liquid receiver or receiver is positioned in the liquid line between the condenser and the expansion valve. The receiver may be installed in a branch in the liquid line so that the flow from the condenser to the expansion valve bypasses the receiver, or in an alternate construction the receiver may be installed so that the full flow from the condenser to the expansion valve traverses the receiver. PA1 a liquid sight glass positioned in the liquid line before the TXV. The liquid sight glass is a fitting generally designed to accommodate the full liquid flow to the TXV. The fitting has a transparent window allowing the operator to see the condition of the liquid flowing. If there is a receiver in the liquid line, the sight glass is positioned in the liquid line between the receiver and the TXV. PA1 (1) Low refrigerant level in the system receiver. This is an alarm situation indicative of a leak in the refrigeration piping system and a loss of refrigerant. PA1 (2) Insufficient condensing pressure. The pressure of the flowing liquid refrigerant is at a saturation temperature which is below the ambient temperature surrounding the piping. This causes a boiling of the liquid refrigerant. This situation can require either a control action or an alarm or both. PA1 (3) Excessive pressure drop in liquid pressure caused by a rise in liquid line elevation or a restriction in receiver or liquid line or a plugged filter-drier.
The correct operation of refrigeration system requires that liquid flowing to the expansion valve (TXV) be substantially bubble free. Bubbles in the liquid stream flowing to the TXV severely degrade its operation and the operation, capacity and reliability of the whole system. In order to enable an operator or service person to observe whether there are bubbles in the liquid flowing to the TXV there is generally provided a device called
The presence of bubbles in the liquid stream flowing to the TXV generally is an indication of some system problem. Depending on the size and persistence of the bubbles the problem may be minor, not requiring any action, or serious requiring immediate action.
Vapor bubbles can exist in the flowing liquid refrigerant for short periods of time without causing excessive cooling capacity loss. However, a continuous stream of vapor bubbles mixed with the flowing liquid can seriously degrade system capacity thereby allowing the cooled product or environment to become warm. The consequences of degraded system capacity depend on the product or environment cooled. For example: if food, spoilage; if human, discomfort, dissatisfaction and loss of production; if computers, catastrophic loss of data and shut-down.
Further, if the refrigerant is a halogen type known as a CFC or HCFC, the loss to the atmosphere could contribute to serious environmental problems including loss of stratospheric ozone and increasing the earth temperature via the so-called greenhouse effect.
Persistent bubbles in the refrigeration system liquid line first suggest one of the following conditions:
A Bubble detector of the present invention positioned at one or more points within the liquid flow stream can be employed to distinguish between a bubbling and an bubble-free condition of the liquid flow stream and to provide an alarm or to take some corrective measure.
There are other applications in refrigeration for a bubble detector besides the liquid line application described in the preceding paragraphs.
For example, a bubble detector can be used to determine the end of defrost for reverse flow hot gas defrost. This type of defrost is common on multi-compressor supermarket refrigeration systems. The bubble detector would be used to monitor the condensed defrost gas flowing from the evaporator and returning to the liquid line and to terminate the defrost when bubbles appear.
A bubble detector could also be positioned in the liquid line between the condenser and the receiver to monitor the liquid flow stream entering the receiver. That flow stream normally is not pure bubble-free liquid but has liquid which includes many bubbles. In the abnormal case, where the refrigeration system is overcharged, that flow stream becomes bubble-free. A bubble detector position in the liquid line at the receiver inlet, therefore could be employed to diagnose a condition of system overcharge.
In the systems where there is filter drier installed in the liquid line two bubble detectors would be installed in the liquid line, one immediately before the filter drier, the inlet detector, the second, immediately after the filter drier, the outlet detector.
The condition of bubbles at the outlet detector coincident with a condition of no bubbles at the inlet detector would trigger an alarm condition warning of a plugged filter drier or a drier having excessive pressure drop for the subcooling available.
The bubble detector of the present invention utilizes the optical principle of refraction and the optical principle of total internal reflection to distinguish between the presence of liquid and vapor at an optical interface at which a light beams is directed.
Bubble detectors which utilize the principles of total internal reflection and which depend on refractive differences between a liquid and a gas of a monitored fluid stream are taught in Kramer's U.S. Pat. No. 4,559,454 and in Smith's U.S. Pat. No. 4,859,864.
The transition between refraction and total internal reflection relies on differences in refractive index between the liquid phase and the vapor phase of the monitored fluid and on the angle of light from a source impinging on an interface between a transparent window having a refractive index and the monitored fluid, whereby a condition of total internal reflection exists when vapor only occupies the interface, and partial reflection and partial transmission with refraction when liquid only occupies the same interface. The condition of total internal reflection arises when light moves from a first transparent medium with a first index of refraction through an interface at an angle to a perpendicular to the interface erected in the first medium, to a second transparent medium with a second index of refraction and the angle to the perpendicular is greater than a critical angle. The angle is expressed by an equation to be defined later.
Under conditions of total internal reflection the internal surface of the optical window at the interface between the window and the monitored fluid acts like a high efficiency silver mirror. In order to employ the transition between total internal reflection and refraction for the purpose of sensing the difference between liquid and vapor refrigerant, the optical construction must be such that the angle of incident light rays striking the interface between the window and the monitored fluid must be greater than the critical angle of incidence for vapor and less than the critical angle of incidence for the liquid refrigerant.
The critical angle of incidence is the angle between a perpendicular erected to the interface between the glass and the refrigerant, and a ray of light moving within the glass toward the refrigerant-glass interface. The critical angle is the special case where the ray of light, having left the glass, has a path in the refrigerant which is parallel with the surface of the glass. Light moving within the glass toward the interface with the refrigerant at an angle greater than the critical angle is totally reflected back into the glass. Hence, the critical angle is also known as the angle for total internal reflection. The critical angle is calculated using Snell's law (Willebrord Snell 1621) from the formula ##EQU1## The table shown below lists the index of refraction and the critical angle of incidence for various liquid and gaseous refrigerants, relative to a typical glass having an index of Refraction of 1.517.
TABLE 1 ______________________________________ Critical Angle Of Incidence Medium Index (Degrees) ______________________________________ Gas 1.000 41.2 (for refrigerants vapors, air and vacuum) Liquid R11 1.362 63.9 R12 1.288 58.1 R22 1.234 54.4 R113 1.357 63.5 R114 1.294 58.5 R502 1.234 54.4 R717 1.325 60.9 ______________________________________
Materials other than glass with similar optical properties could be used as well.
Examination of the table shows that for the liquid refrigerants the smallest critical angle is 54.4.degree. while for all the gases the critical angle is 41.2.degree.. The present invention teaches an apparatus and method which employs the principle of total internal reflection together with a bubble accumulating chamber positioned in a flow stream of liquid refrigerant to monitor the flow stream and to initiate action when the accumulation of bubbles reaches a significant level.